neonet/newlink/DialLink: Fix SIGSEGV in case client handshake fails

In case the last 'handshakeClient' call returns an error, 'DialLink'
returns 'link = nil, err = nil'. Callers of 'DialLink' then don't
recognize that 'link' is 'nil', as it's the convention to only check if
'err' is 'nil', which leads to a 'segmentation violation' as soon as
subsequent code tries to access fields of 'link':

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x14 pc=0x7087ae]

goroutine 5 [running]:
lab.nexedi.com/kirr/neo/go/neo/neonet.(*NodeLink).NewConn(0x0)
	/srv/slapgrid/slappart82/srv/runner/instance/slappart6/software_release/parts/wendelin.core/wcfs/neo/go/neo/neonet/connection.go:404 +0x4e
lab.nexedi.com/kirr/neo/go/neo/xneo.Dial.func1()
	/srv/slapgrid/slappart82/srv/runner/instance/slappart6/software_release/parts/wendelin.core/wcfs/neo/go/neo/xneo/connect.go:138 +0x52
lab.nexedi.com/kirr/neo/go/internal/xio.WithCloseOnErrCancel.func2()
	/srv/slapgrid/slappart82/srv/runner/instance/slappart6/software_release/parts/wendelin.core/wcfs/neo/go/internal/xio/xio.go:114 +0x6a
created by lab.nexedi.com/kirr/neo/go/internal/xio.WithCloseOnErrCancel in goroutine 21
	/srv/slapgrid/slappart82/srv/runner/instance/slappart6/software_release/parts/wendelin.core/wcfs/neo/go/internal/xio/xio.go:109 +0x1ad
```

This patch fixes this issue so that now 'err' and 'link' are never both
'nil' again.

go/neo/neonet/newlink.go

-// Copyright (C) 2016-2023  Nexedi SA and Contributors.
+// Copyright (C) 2016-2024  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -319,9 +319,10 @@ func init() {
 
 // DialLink connects to address on given network, performs NEO protocol
 // handshake and wraps the connection as NodeLink.
-func DialLink(ctx context.Context, net xnet.Networker, addr string) (link *NodeLink, err error) {
+func DialLink(ctx context.Context, networker xnet.Networker, addr string) (link *NodeLink, err error) {
 	for _, enc := range dialEncTryOrder {
-		peerConn, err := net.Dial(ctx, addr)
+		var peerConn net.Conn
+		peerConn, err = networker.Dial(ctx, addr)
 		if err != nil {
 			return nil, err
 		}